OBJECTIVE: To reassess airborne pollen counts and to study spore counts by volumetric collection in Rochester, Minnesota.MATERIAL AND METHODS: Samples of pollen and spores were collected from the top of a six-story building in downtown Rochester by use of a rotational rod sampler, which was activated for 30 seconds every 10 minutes for 24 hours a day, 5 days a week, from mid-April to mid-October 1992 through 1995. Particles were stained with Calberla's fluid and viewed microscopically by transmitted light. Identifications were substantiated by comparison with field-collected "known" materials and standard reference guides. Data were expressed as "particles retained/cubic meter of air sampled."RESULTS: Because the methods of collection varied, only qualitative comparisons could be made with a study done in Rochester more than 3 1/2 decades earlier. For tree pollens, a similar progression of appearance and seasonal duration were noted for most species. Alder (Alnus), willow (Salix), and hickory-pecan (Carya), however, were found to have distinctly longer seasons, and walnut (Julgans) was detected 2 weeks earlier in the season. The most frequently detected tree pollen had changed from oak (Quercus) to apple and crab apple (Malus), which was not reported at all previously. Ragweed (Ambrosia) was still found to peak at the end of August and had a similar duration. Study of grass and nettle pollen as well as the Amaranthaceae-Chenopodiaceae complex yielded findings similar to those in the prior investigation. Fungal spores were detected from April through October and represented a substantial percentage of the airborne allergens; Cladosporium, ascospores, and basidiospores were the most common.CONCLUSION: Volumetric assessment of airborne pollen and spore levels in Rochester, Minnesota, yielded useful information for clinicians in southern Minnesota that could help direct skin test selection.